A well-known technique for providing multi-point modulation over a single communications channel is Time Domain Duplexing (TDD). In TDD, a control modem transmits data downstream to one or more tributary modems, then the control modem receives transmissions upstream from any of the tributary modems on a shared channel. If the downstream and upstream channels are isolated (spatially, by frequency division, or by other means) then the downstream transmission can be continuous even though the upstream channel is shared. Each modem must terminate its transmission to allow other modems sharing the same channel to transmit. Examples of TDD systems include Multiple Virtual Line, Digital Subscriber Line (DSL) and Reach DSL®.
Block framed burst communication systems, such as DSL Discrete Multitone Modulation (DMT), also terminate transmission. These systems terminate transmission at the end of each DMT symbol period.
TDD systems typically used uncoded modulation rather than Trellis Coded Modulation (TCM) even though TCM provides 3-6 dB of performance gain, because Trellis decoding significantly increases line turn-around time (the time it takes a particular tributary modem to stop receiving and start transmitting). Trellis decoding typically requires a lengthy delay through a Viterbi decoder. During this time, the channel is out of service because the receiver must complete Viterbi decoding of a frame before transmission.
This same Viterbi decoder delay makes Trellis coding incompatible with use of a Decision Feedback Equalizer (DFE) in the receiver, since decoder delay prevents timely generation of reference vectors needed by the DFE. Therefore, in systems using TCM, intersymbol interference is typically handled with a precoder in the transmitter rather than a DFE in the receiver. However, conventional precoder designs are incompatible with systems that switch constellation densities or that switch between coded/uncoded transmissions (e.g. Reach DSL V2®). With a conventional precoder, power is scaled at the precoder output, and the power of the signal within the precoder's Finite Impulse Response (FIR) varies directly in proportion to constellation density. A change in constellation density, as would occur for a change in data transmission rate, therefore causes a power discontinuity within the precoder, which results in errors.